Limits

Regeneration

This ability does not move displaced parts of the body back into the correct location. After filming the videotape, Claire had to manually re-insert her displaced ribs before the flesh on her ribcage would heal (Genesis), and Peter Petrelli had to manually rotate his broken limbs to the correct orientation before it would heal when he used his empathic mimicry to mimic Claire's ability (Homecoming). However, Adam Monroe was able to completely heal from a massive explosion at Whitebeard's camp. Though not able to heal immediately after experiencing the explosion, as evidenced by his charred body in the ruins of the tents (Out of Time). In a similar circumstance Claire survived radioactive explosion twice healing immediately from severe burns after in both instances in the actual timeline as well as in the exposed future. Claire has also shown to be unaffected by Electrocution healing damage with ease.

Typically, Claire's blood does not return to its place in her body when an associated wound heals, although it can if the wound is small and her blood has not spread far from it (the only example being when her brother stapled her hand). She is consistently covered with blood following a serious injury.

As demonstrated for the first time in Lizards, Claire is able to regrow body parts that have been removed. She experimented with this for the first time by cutting off her little toe with a pair of scissors, then watching it grow back with much surprise. In Resistance, Future Claire's arm is blown off by a grenade and she re-grows it.

Claire certainly feels her injuries--she cried out from having a staple put into her hand and winced in pain at other times--but the pain does not incapacitate her in the way it would an ordinary person. She has stated that the pain she feels from injuries diminishes very rapidly (Kindred). However, since having her brain examined by Sylar (The Second Coming), Claire has stated that she no longer feels pain of any kind, although when demonstrating the power to Gretchen, she pretended she did for a second.

Peter has shown that this power has the capability to reverse memory damage caused by the Haitian. However, unlike most instances of the ability's usage, memory damage reversal requires active concentration (Four Months Ago...).

It's revealed that it can also heal nervous system damage. Arthur Petrelli was completely paralyzed and breathing through a tube having suffered permanent damage to his nervous system due to Angela poisoning him. He absorbed Adam Monroe's power, killing Adam, but healing all of the damage done to his body by the poison, reversing his paralysis.

This power also speeds the recovery from inebriation or cancels out its effects all together, as Claire briefly mentioned that her liver tissue is rapidly regenerating (Into Asylum).

While the ability protects an individual from infections and other pathogens, it works independently from one's immune system: when Claire was shot while her ability wasn't working, her immune system was too weak to fight off infections, leading to a system wide critical condition that lead to her death. When her ability started working again, she came back to life. (The Eclipse, Part 2)

Regeneration does not work correctly if a foreign object obstructs the wound. This is especially true of foreign objects lodged in a particular spot in the brain. This limitation is presumably related to the importance of the brain in the nature of evolved humans, as theorized by Chandra Suresh (One Giant Leap). There are many examples of this limitation and knowledge thereof;

After Brody "killed" Claire, she did not begin to regenerate until the coroner removed the tree branch lodged in her brain. (Collision)

Peter experienced a similar situation with a piece of glass in the back of his head (.07%).

However, in The Second Coming, Sylar not only leaves her brain in place, but explains with his new understanding of her powers, apparently through his intuitive aptitude, he claims that he cannot kill her and she could never die.

Angela sent Peter and the Haitian to kill Arthur by shooting him through the back of the head otherwise he could regenerate. Ultimately the bullet Peter fired was telekinetically shoved all the way through Arthur's head by Sylar, presumably completely destroying the spot and killing him permanently. (Our Father)

Claire uses knowledge of this spot in the brain to incapacitate Sylar with a shard of glass (Dual).

Danko and his team also had this knowledge, claiming that a bullet to the spot fired at either Claire or Sylar would kill them (Trust and Blood).

Sylar claimed that this spot was microscopic, requiring one to either be a good shot or have intimate knowledge of where it's located. He also claimed that this spot could be moved, apparently doing so to avoid death by Danko by knife, when hit where the spot should have been. (I Am Sylar)

Despite this limitation, regenerators can expel small objects (such as a bullet) lodged in her body as long as they are not disrupting their brain functions (Company Man, Kindred).

In several episodes it has been stated or shown that an individual with the ability can be permanently killed. It is stated by Adam Monroe that if a person with the power is decapitated or the brain is severally damaged the ability to heal is canceled out and leaving the individual permanently dead (Out of Time)(Truth & Consequences).

In one instance Adam died due to aging rapidly, when his power was taken from him by Arthur. In another instance the power was canceled out by the Haitians ability to negate powers, which resulted in the death of Arthur when Peter went to shoot him in the head, but the bullet was subsequently stopped in mid-air by Sylar, who then allows the bullet to continue on its course, killing Arthur while he is in his momentarily depowered state (Our Father). A similar circumstance is seen when the Future Claire shoots Future Peter in the exposed future in partnership with the Future Haitian, using his ability negation to make sure Peter's abilities won't allow him to escape as well as allowing him to be seriously wounded from the bullet, as he is unable to regenerate (I Am Become Death). After giving birth to Tommy Clark and Malina, Claire died of cardiac arrest from a traumatic seizure. To the shock of her father, she did not regenerate despite not having weak spot blocked. (June 13th, Part One) Its later revealed that her son inherited Arthur Petrelli's powers resulting in him draining her ability as she gave birth to him. Without her powers, Claire was able to die a normal death. (June 13th, Part Two)

Regenerators are not immune to being frozen (Brother's Keeper). However, should any of their limbs shatter, they will regenerate them as normal.

Anti-aging

Adam Monroe lived for centuries without aging significantly. Angela Petrelli confirms that this is an effect of his ability (Cautionary Tales). She tells Matt Parkman that after a certain amount of regeneration, the body simply stops aging. After Arthur Petrelliremoved this ability from him, Adam aged rapidly, crumbling to dust within a few seconds, demonstrating that without his power continually repairing age-related damage, he would quickly die.

Adam injected a small amount of his blood into Nathan's intravenous drip, causing Nathan to immediately start to heal from his radiation burns (Four Months Ago...). Adam did not show any worry about the possible adverse effects of mixing blood types.

Unlike Adam, a recipient of Adam's blood cannot halt the aging process indefinitely. Adam's blood cured his wife Diane of tuberculosis, but she still died "peacefully" twenty years later. (The Ten Brides of Takezo Kensei)

According to Noah, the regenerative properties of a blood transfusion would not heal tumors. Because they are living tissue, regeneration would cause them to grow larger. (Tabula Rasa)

Claire and her mother mention an incident at the dog park in which a "big dog was gonna chew [Claire's] hand off." Claire says that "it looked worse than it was." (Better Halves)

Claire's healing ability does not seem to affect her pierced ears. This presumably results from the earring posts obstructing the wounds, although it's not clear whether she has to "re-pierce" her ears whenever she removes her earrings.

In a Super Bowl XLIII commercial where several Heroes suit up in football uniforms to battle against NFL stars, Claire, as the cheerleader of course, uses her ability to regenerate after being crushed by the other team.

According to Jack Coleman's blog, Jack explains why they didn't used Claire's blood on Nathan when he died. "This time around, there was no Bob or Suresh on the scene, there was no bag o' blood waiting to be utilized, there was no time. Nathan's throat was slit wide open and he bled out".

Lauren calls Claire a "regen".

For some reason, when Claire gave birth to Tommy Clark and Malina, she did not regenerate when she died of cardiac arrest from a traumatic seizure. Her weak spot was not blocked, but her power seemed to cease to function when she gave birth. Angela later realizes that Tommy inadvertently drained this power from her as he inherited a version of Arthur Petrelli's abilities which resulted in her permanently dying without her ability to revive her.

At Homecoming, regeneration was one of several abilities Peter was exposed to but the only one he mimicked that night. It was later the first ability Peter consciously called upon when learning to access his absorbed abilities, albeit inadvertently.

After Peter starts accessing his absorbed powers, his regeneration seems to be constantly active like with Claire and not an ability he has to call upon like his other absorbed abilities. When Peter is shot by Will he regenerates despite lacking the memory of Claire to call upon the ability. He also heals from the injuries Ricky's gang gave him without even noticing or realizing it.

Trivia

The exact nature of Claire not being able to experience pain is unknown. In real life, there is a medical condition called hypoalgesia (aka hypalgesia), which notes a decreased sensitivity to painful stimuli and potential loss of the ability to distinguish between temperatures. Some people are born with a severe case of the disorder, called congenital insensitivity to pain with anhidrosis (aka CIPA, congenital analgia). The anhidrosis means that the person cannot sweat, although Claire was able to sweat while being interrogated by Meredith (One of Us, One of Them).

If this power is deactivated Claire can feel pain as shown during The Eclipse and when Peter was accidentally blocking her powers while he possessed mental manipulation and she cut herself and felt the pain.